CERAVER's Document FR-A-2 No. 2436958 describes a heat exchanger unit with parallel ducts obtained by extrusion in which the ends of a first series of ducts which are to be kept open are temporarily masked by detachable screens of flexible material, the end of the heat exchange unit is dipped in viscous slip which is thereby caused to enter the ends of a second series of ducts which are not masked, the heat exchange unit is fired so as to fix the slip which enters the ends of said second series of ducts and the detachable screens are removed to uncover the openings of the ducts of the first series. Such a method takes a relatively long time since it requires each row of ducts whose ends are to be kept open to be masked temporarily and it is difficult to apply it to structures with ducts of small transversal dimensions.
Preferred embodiments of the present invention provide a method of closing the ends of selected ducts which is easier and quicker to put into practice and which can be applied without difficulty even to ducts of small transversal cross-section, i.e. a few millimeters or even less.